Method Of Calibrating Impedance Measurements Of A Battery

ABSTRACT

A method of calibration is described that simplifies the measurement of battery impedance conducted in-situ while determining battery state-of-health. A single shunt measurement with a known Sum of Sines (SOS) current, at the desired frequency spread and known root mean squared (RMS) current is used to create a calibration archive. A calibration selected from this archive is used to calibrate an impedance measurement made on the battery.

This application claims benefit of U.S. Provisional Application No. 62/326,923, filed Apr. 25, 2016, and U.S. Provisional Application No. 62/331,730, filed May 4, 2016, the disclosures of which are hereby incorporated by reference in their entirety including all figures, tables and drawings.

BACKGROUND OF THE INVENTION

Batteries and other electrical energy storage devices have become widely used in not only military, space, and commercial applications but also in domestic applications. Therefore, it has become even more important to be able to efficiently and effectively obtain an accurate estimation of the battery's state-of-health. While voltage, current, and temperature may be used to gauge the remaining capacity of a battery, in critical applications it is also necessary to know impedance and power capability to get an accurate picture of battery health. Ideally, any measurement of battery health is done in-situ and has minimal impact on the battery. A great deal of work has been conducted to test battery impedance without effecting battery status. This work is documented in, for example, U.S. Pat. Nos. 7,688,036; 7,395,163 B1; 7,675,293 B2; 8,150,643 B1; 8,352,204 B2; 8,762,109 B2; 8,868,363 B2; and 9,244,130 B2, and U.S. Published Patent Application Nos. 2011/0270559 A1; 2014/0358462 A1; and 2017/0003354 A1. Each variation of the methods described in these documents improve the process of assessing battery health by, for example, increasing resolution. Recently, a method for testing battery impedance has been described that increases the resolution of a known system by a factor often. Key features of this high resolution method involve a new algorithm, auto-ranging to obtain the optimum level of excitation current, and increased preamplifier gain. The method also required an additional measurement channel that captures time records of the Sum-Of-Sines (SOS) current in addition to the SOS voltage from the test battery.

Although the above-methods have refined this important process, an improved method for calibration that will greatly simplify the calibration process and eliminate the extra measurement channel needed for some methods is still needed.

All patents, patent applications, provisional patent applications and publications referred to or cited herein, are incorporated by reference in their entirety to the extent they are not inconsistent with the teachings of the specification.

SUMMARY OF THE INVENTION

The invention involves an improved method of calibrating impedance measurements of a battery. The method needs only a single measurement with a known Sum of Sines (SOS) current, at the desired frequency spread and known root mean squared (RMS) current.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows the single shunt calibration of a test cell at 500 mA using the method of the subject invention.

FIG. 2 shows the single shunt calibration of a test cell with RMS current scale 500 mA down to 250 mA using the method of the subject invention.

FIG. 3 shows the single shunt calibration of a test cell with RMS current scale 500 mA down to 125 mA using the method of the subject invention.

FIG. 4 shows the single shunt calibration of a test cell with RMS current scale 500 mA down to 62.5 mA using the method of the subject invention.

FIG. 5 shows a single shunt calibration with Solatron EIS of a test cell using the method of the subject invention.

FIG. 6 shows a single shunt calibration with Solatron EIS of a test cell using the method of the subject invention.

FIG. 7 shows a single shunt calibration with Solatron EIS of a test cell using the method of the subject invention).

FIG. 8 shows a lead acid battery measured at 62.5 mA and 15 frequency SOS showing 18 to 15 frequency and 500 mA to 62.5 mA RMS current and frequency scaling.

FIG. 9 shows a lead acid battery measured at 125 mA RMS SOS showing 18 to 15 frequency and 500 mA to 125 mA RMS current scaling and frequency scaling.

FIG. 10 shows splining the calibration 14 frequencies in octaves downward from 1000 Hz calibrated with 15 frequencies in octaves from 0.1 Hz to 1638.4 Hz.

FIG. 11A demonstrates saturation tolerance time CTC algorithm with a 12 V lead acid car battery and shows the unclipped battery time record.

FIG. 11B demonstrates saturation tolerance time CTC algorithm with a 12 V lead acid car battery and shows the unclipped TCTC with baseline HCSD spectrum.

FIG. 11C demonstrates saturation tolerance time CTC algorithm with a 12 V lead acid car battery and shows the clipped battery time record.

FIG. 11D demonstrates saturation tolerance time CTC algorithm with a 12 V lead acid car battery and shows the clipped spectrum with baseline HCSD spectrum.

FIG. 12A demonstrates saturation tolerance TCTC algorithm with a Lithium ion battery and shows the unclipped battery time record.

FIG. 12B demonstrates saturation tolerance TCTC algorithm with a Lithium ion battery and shows the unclipped TCTC with baseline HCSD spectrum.

FIG. 12C demonstrates saturation tolerance TCTC algorithm with a Lithium ion battery and shows the clipped battery time record.

FIG. 12D demonstrates saturation tolerance TCTC algorithm with a Lithium ion battery and shows the clipped spectrum with baseline HCSD spectrum.

FIG. 13 is a block flow diagram of a method for generating a measure of impedance of a test device.

DETAILED DESCRIPTIONS OF THE INVENTION

The method of the subject invention is involves single shunt calibration (SSC) that applies to all generations of (Impedance Measurement Box) IMB. The subject method simplifies use of the IMB to assess battery health. The Idaho National Laboratory (INL) has described the design and construction of the IMB in numerous patent documents (see, for example, U.S. Pat. Nos. 7,688,036; 7,395,163 B1; 7,675,293 B2; 8,150,643 B1; 8,352,204 B2; 8,762,109 B2; 8,868,363 B2; and 9,244,130 B2, and U.S. Published Patent Application Nos. 2011/0270559 A1; 2014/0358462 A1; and 2017/0003354 A1). Spectrum algorithms used in the implementation are also described in the above patent documents and include, but are not limited to, harmonic compensated synchronous detection (HCSD), fast summation transformation (FST), generalized fast summation transformation (GFST), frequency cross talk compensation (FCTC), time cross talk compensation (TCTC), harmonic orthogonal synchronous transformation (HOST). Each of these spectrum algorithms are special cases of a rapid Fourier Transform that bring the measurement time record captured by the IMB measurement into the frequency domain at only the frequencies that were part of the IMB excitation signal to the test battery. The calibration in the present generation 50V IMB (U.S. Patent Application Publication No. 2014/0358462) is accomplished by a complicated measurement scheme which uses 3 different shunts to generate calibration constants that yield a very accurate measurement of the impedance spectra from a test battery (Morrison, William. H., thesis, 2012). In contrast, the claimed method requires only a single measurement with a known Sum Of Sines (SOS) current, at the desired frequency spread and known RMS current.

As an example consider application to the 50V IMB (U.S. Patent Application Publication No. 2014/0358462). With the present 50V IMB HCSD algorithm system (U.S. Patent Application Publication No. 2014/0358462), the calibration for a given SOS frequency spread (octave harmonic short 0.1 Hz to 1638.4 Hz or long 0.0125 Hz to 1638.4 Hz) and a given SOS RMS current, the measurement time record that is processed into the frequency domain is typically one period of the lowest frequency. As part of the calibration the SOS current output is pre-emphasized to mitigate the IMB system frequency response. Additionally, the 3 shunt calibration scheme computes gain and offset constants for both magnitude and phase at each frequency. Equation 1 represents the time record captured by the IMB from a measurement on a test battery.

V _(B)(t)=I _(SOS)(t)*A _(S)(t)*Z _(B)(t)  (1)

Where: I_(SOS)(t) is the SOS current time record

-   -   A_(S)(t) is the measurement system impulse response     -   Z_(B)(t) is the test battery impedance impulse response

The * in Equation 1 is a convolution operation. Because of the calibration pre-emphasis, I_(SOS)(t) is given by:

$\begin{matrix} {{I_{SOS}(t)} = {{RMS}\sqrt{\frac{2}{m}}{\sum\limits_{i = 1}^{m}{\sin \left( {\omega_{i}t} \right)}}}} & (2) \end{matrix}$

Where: RMS is the RMS of the SOS current

-   -   m is the number of frequencies in the SOS     -   ω_(i) is the i^(th) radian frequency         Note that the RMS of a SOS signal is:

${RMS} = {\sqrt{\sum\limits_{i = 1}^{m}\left( {\frac{1}{T_{i}}{\int_{T_{i}}{\left( {V_{P}\mspace{11mu} {\sin \left( {\omega_{i}t} \right)}} \right)^{2}{dt}}}} \right)} = {V_{P}\sqrt{\frac{m}{2}}}}$ $V_{P} = {{RMS}\sqrt{\frac{2}{m}}}$

Equations 1 and 2 brought into the frequency domain via the 50V IMB HCSD algorithm (Morrison, William H., thesis, 2012) becomes:

$\begin{matrix} {{\overset{\rightarrow}{V}}_{Bi} = {{RMS}\sqrt{\frac{2}{m}}\left( {A_{Si}\bullet \mspace{11mu} \varphi_{Si}} \right)\left( {Z_{Bi}\bullet \mspace{11mu} \varphi_{Bi}} \right)}} & (3) \end{matrix}$

Where: A_(Si)□ϕ_(Si) the measurement system frequency response at the i^(th) frequency

-   -   Z_(Bi)□ϕ_(Bi) is the desired battery impedance at the i^(th)         frequency         The effect of calibration is to multiply Equation 3 by a         calibration constant given by:

$\begin{matrix} {{CAL}_{i} = \left( {{RMS}\sqrt{\frac{2}{m}}\left( {A_{Si}\bullet \mspace{11mu} \varphi_{Si}} \right)} \right)^{- 1}} & (4) \end{matrix}$

Clearly the calibration applied to Equation 3 results in the desired battery impedance and the 50V IMB has demonstrated this with great success via the 3 shunt magnitude calibration and the stepped phase shift calibration both yielding gain offset calibration constants that represent Equation 4 (Morrison, William H. thesis, 2012). Observe that Equation 4 is a calibration constant that is a combination of SOS current pre-emphasis and magnitude phase calibration at each frequency. The subject method does everything in a single measurement with a single shunt, single shunt calibration (SSC).

For the 50V IMB system the concept is very simple. The system will perform a spectrum measurement on a known non-inductive shunt for example a 50 mOhm non-inductive calibration shunt (as shown in FIG. 13, Block B1). The SOS current is set to the high level, 500 mA RMS (as shown in FIG. 13, Block B2). The system must be configured for no pre-emphasis and no calibration (U.S. Patent Application Publication No. 2014/0358462). As the 50V IMB uses the HCSD algorithm, either 18 frequencies are selected, 0.0125 Hz to 1638.4 Hz or 15 frequencies 0.1 Hz to 1638.4 Hz both in octave steps (as shown in FIG. 13, Block B3). The measurement is performed and processed via the HCSD algorithm to convert the captured time record into the frequency domain (as shown in FIG. 13, Blocks B4 and B5 respectively). Then the complex results at each frequency are normalized to the measurement shunt value and the SOS RMS current (as shown in FIG. 13, Block B6). These results are in fact Equation 6 and are stored in a calibration file that when recalled can be used to calibrate a battery impedance measurement with the same frequency spread and different RMS current (as per Equation 9)(as shown in FIG. 13, Block B7).

For the single shunt calibration (SSC), we assume that single shunt used is constant and independent of frequency over the frequency range of the IMB. Additionally, all measurements are made without any pre-emphasis. Thus as a function of time the IMB measurement of that shunt V_(SHUNT)(RMS,iΔt) is given by Equation 5.

V _(SHUNT)(RMS,iΔt)=V _(SOS)(RMS,iΔt)*H _(OUT)(t)*R _(SHUNT) *H _(IN)(iΔt)   (5)

Where *: indicates the convolution operation

-   -   V_(SOS)(RMS,iΔt): is the computer generated signal to IMB         current drivers     -   H_(OUT)(t): is the current driver system response in time     -   iΔt: is the computer discrete time     -   R_(SHUNT): is the calibration shunt, ohms     -   H_(IN)(iΔt): is the IMB system input response in discrete time         Also as a function of time the IMB measurement of a test battery         is given by Equation 6.

V _(Meas)(RMS,iΔt)=V _(SOS)(RMS,iΔt)*H _(OUT)(t)*Z _(BAT)(t)*H _(IN)(iΔt)  (6)

Where: Z_(BAT)(t) is the impedance impulse response of the battery as a function of time.

For the SSC the time record of the shunt (Equation 5) is processed by the HCSD algorithm of the IMB, normalized by R_(SHUNT) and stored as calibration. Equation 7 illustrated the shunt time record brought into the frequency domain at one of the SOS frequencies ω_(i).

V _(SHUNT)(ω_(i))=V _(SOS)(RMS,ω_(i))H _(OUT)(ω_(i))R _(SHUNT) H _(IN)(ω_(i))  (7)

Where: ω_(i) is radians/sec

Note that the convolution operation in Equation 5 goes to multiplication in Equation 7. The time record of the battery given by Equation 6 when brought into the frequency domain at one of the SOS frequencies ω_(i) is given by Equation 8.

V _(Meas)(ω_(i))=V _(SOS)(RMS,ω_(i))H _(OUT)(ω_(i))Z _(BAT)(ω_(i))H _(IN)(ω_(i))   (8)

Performing division in the frequency domain the essence of calibration is given by Equation 9.

$\begin{matrix} {{Z_{BAT}\left( \omega_{i} \right)} = {R_{SHUNT}\frac{{V_{SOS}\left( {{RMS},\omega_{i}} \right)}{H_{OUT}\left( \omega_{i} \right)}{Z_{BAT}\left( \omega_{i} \right)}{H_{in}\left( \omega_{i} \right)}}{{V_{SOS}\left( {{RMS},\omega_{i}} \right)}{H_{OUT}\left( \omega_{i} \right)}R_{SHUNT}{H_{in}\left( {i\; \Delta \; t} \right)}}}} & (9) \end{matrix}$

Thus the SSC is a collection of measurements of R_(SHUNT) at standardized RMS currents and SOS frequency spreads brought into the frequency domain by the HCSD algorithm. For the IMB there are 2 standardized frequency ranges and 4 standardized RMS currents. To calibrate for this, results in 8 measurements with the single shunt for SSC which are performed fully automated with a single shunt hook-up. A vast improvement over the original manual 3 shunt calibration process.

Observe Equation 7, if in addition to being normalized to the shunt if it were normalized also to the calibration RMS current it can be used as a calibration for any battery measurement RMS current by scaling it to that measurement RMS current.

Example 1—Validation Using 50V IMB and RMS Current Scaling

The 50V IMB at Montana Tech of the University of Montana (Butte, Mont.) was used for initial testing. A long run frequency (0.0125 Hz to 1638.4 Hz) domain calibration file was generated (via HCSD) with-out any calibration or pre-emphasis at an SOS current of 500 mA and a 50 mOhm shunt (as shown in FIG. 13, Blocks B1-B7). It was normalized to the 50 mOhm shunt and the RMS current. Time records of measurements (long runs, 0.0125 Hz to 1638.4 Hz) were made and captured for Test Cell (TC) #3 (Morrison, William H., thesis, 2012) at RMS currents of 500 mA, 250 mA, 125 mA and 62.5 mA again without any pre-emphasis (as shown in FIG. 13, Blocks B8 and B9). All those Test Cell time records were post processed into the frequency domain with the HCSD algorithm ((as shown in FIG. 13, Block B10). The frequency domain calibration was scaled to each measurement current RMS ((as shown in FIG. 13, Block B11). Test cell measurements were made at all the different currents (of 62.5 mA, 125 mA, 250 mA and 500 mA) and calibrated per Equation 9. The results are shown in FIGS. 1-4. The detected TC #3 spectra are plotted with Electro-chemical impedance spectroscopy (EIS) (Solartron Analytical, 2012) data by INL on TC #3. The validity of the subject method is realized by the degree of coincidence of the two plots. Like results were achieved when the subject method was applied to Test Cells #3, #5, and #7. Results demonstrating SSC with these test cells are plotted and shown in FIGS. 5-7.

These results show that 500 mA shunt data can reach all the way down the 62.5 mA to capture the spectra of TC #3 and the results match closely with INL EIS (Solartron Analytical, 2012) data for TC #3 (as shown in FIG. 13, Block B12). This means that for calibration all that is needed is frequency domain files of known shunt, known frequencies and that file will work on measurements with the same frequencies and different known current with good results. This is a significant enhancement to the existing 50V IMB.

As stated previously, with the single shunt calibration, with standard RMS currents and standard frequency ranges, a calibration is fully automated with as few as 8 measurements. Never the less, that can be reduced to a single calibration measurement with frequency scaling and RMS current scaling. In examining Equation 5 for calibration it would be normalized to the calibration RMS current and the shunt value. Then for a calibration it would be scaled by the measurement RMS. Consider the RMS of an SOS:

$\begin{matrix} {{{RMS}({SOS})} = {\sqrt{\left( {\sum\limits_{m}\frac{V_{P}^{2}}{2}} \right)} = {V_{P}\sqrt{\frac{M}{2}}}}} & (10) \end{matrix}$

Where: M is the number of frequencies

-   -   V_(P) is the peak voltage of the sine waves in the SOS         Thus given that a measurement frequency range is an octave         harmonic subset of a calibration frequency range the frequency         domain the subset of real and imaginary constants are selected         out and scaled per:

$\begin{matrix} {({Calibration}) \times \sqrt{\frac{M_{CAL}}{M_{MEAS}}} \times \frac{{RMS}_{MEAS}}{{RMS}_{CAL}}} & (11) \end{matrix}$

Example 2—Validation Test of Frequency and RMS Calibration Scaling

Small lead acid battery measured by IMB with 62.5 mA and 15 frequency SOS. IMB spectrum obtained with normal IMB calibration. Uncalibrated time record post processed to the frequency domain and calibrated by an 18 frequency 500 mA shunt time record brought to the frequency domain and scaled to 15 frequency and 62.5 mA RMS. Both spectra are given in FIG. 8. FIG. 9 shows the battery measured by IMB at 125 mA RMS SOS (as shown in FIG. 13, Block B13). These results support frequency and RMS current scaling for SSC.

Example 3—Negative Time to Reinforce the Assumption of Steady State

The fundamental assumption of all IMB data processing algorithms is that the system being measured is in steady state relative to all excitation frequencies. Clearly this is in contradiction to the requirement of performing a rapid measurement. The IMB measurement technique is to excite the test article with a sum of sinusoids with an excitation time record of no more than one period of the lowest frequency. Some researchers using the IMB measurement concept (Waligo, A., 2016) have resorted to using multiple periods of the lowest frequency in order to re-inforce this assumption. A better solution is “Negative Time” (NT), whereby the sum of sinusoids starting at time zero would all be zero but if one goes backwards in time for a fraction of the period of the lowest frequency, then start the excitation there, this has been shown to work very well to establish the steady state approximation (10% is typical) (as shown in FIG. 13, Block B14). This NT portion is either ignored or discarded from the captured voltage response (as shown in FIG. 13, Block B15). Additionally, NT is also needed for shunt calibration as smoothing filter in the IMB will need to be brought to steady state.

When a calibration is scaled the objective is to make V_(P) of a measurement and calibration the same thus the frequency range could be kept standardized as subsets of the calibration frequency range. Never the less, for non-standard subsets, even non-octave harmonic subsets processed via time or frequency CTC (U.S. Pat. No. 8,762,109) the technique of “cubic spline” (U.S. Pat. No. 8,868,363) will select out the calibration constants and they will scale exactly as the above relationship (as shown in FIG. 13, Block B16). FIG. 10 shows splining the calibration of 14 frequencies in octaves downward from 1000 Hz calibrated with 15 frequencies in octaves from 0.1 Hz to 1638.4 Hz (as shown in FIG. 13, Block B17).

Example 4—Application to Saturation Tolerant Time and Frequency CTC

A critical feature of the concept for a High Resolution Impedance Measurement Box (HRIMB) is its ability to digitize signals where the voltage level of the signal is near and occasionally beyond the saturation level of the digitizer within the Data Acquisition system (DAQ). This capability of the HRIMB is realized by replacing the data processing algorithm (HCSD Morrison, W. H., thesis, 2012)) with a variation of time or frequency CTC (U.S. Pat. No. 8,762,109) (TCTC, FCTC). This feature for these 2 algorithms is achieved by examining the captured voltage time record for saturation points (as shown in FIG. 13, Block B18), noting the exact times of saturation, deleting those points from the voltage time record (as shown in FIG. 13, Block B19), and computing the algorithm correction matrices at only the times for non-saturation. This technique works for both TCTC and FCTC. These algorithms will bring the time domain voltage measurements (with occasional saturation) into the frequency domain but they must be calibrated to become impedance. This is achieved by retrieving an archived time record of a single shunt measurement that has the exact frequencies of the “to be calibrated voltage” measurement, has been normalized to the measurement shunt and its RMS current. That time record is scaled to the RMS current of the “to be calibrated voltage” and the exact points of the saturation in “to be calibrated voltage” time record are deleted from this time record (as shown in FIG. 13, Block B20). It is then processed with the same correction matrices used with the “to be calibrated voltage” time record (as shown in FIG. 13, Block B21). The impedance at each frequency is the ratio of the “to be calibrated voltage” phasor to the calibration phasor. Note that if decimation was used on the time records in the application of TCTC or FCTC the same decimation must be applied to the calibration time record. Decimation is a technique of uniformly discarding data points from time records to speed up data processing for TCTC or FCTC. Depending on the type of battery being tested it should be kept below X16 and should be applied prior to deletion of saturated data points.

Demonstration of saturation tolerance Time CTC algorithm with a 12V lead acid car battery, 500 mA RMS SOS current, 15 frequencies (0.1 Hz to 1638.4 Hz) plotted with the IMB HCSD measurement response is shown in FIG. 11. Simulated clipping was done by discarding all points in the time record of the response above or below 0.17V. The algorithm was calibrated by a time record of 15 frequencies and 500 mA RMS applied to a shunt. FIG. 11A is the battery voltage time record. FIG. 11B is the spectrum plotted without clipping plotted with the IMB HCSD spectrum. FIG. 11C shows the clipped at +/−0.17V battery voltage time record. FIG. 11D shows the Time CTC clipped spectrum plotted with the baseline HCSD spectrum.

FIG. 12 shows a similar demonstration of time CTC clipping Li Ion battery. An 18 frequency (0.0125 Hz to 1638.4 Hz) 500 mA RMS SOS measurement was made on an Li Ion battery and a 50.27 mOhm shunt. Clipping tolerance was demonstrated by the battery voltage response at +/−0.2V. The Time CTC results are plotted with the baseline HCSD IMB spectrum. FIG. 12A gives the unclipped battery voltage time record. FIG. 12B gives the time CTC processing of FIG. 12A calibrated with the shunt time record plotted with the HCSD baseline spectrum. FIG. 12C gives the +/−0.2V clipped battery voltage time record plotted with the clipping adjusted shunt time record. FIG. 12D gives the time CTC clipped spectrum plotted with the baseline HCSD spectrum. All time CTC processing was done with a X16 decimation.

It is understood that the foregoing examples are merely illustrative of the present invention. Certain modifications of the articles and/or methods may be made and still achieve the objectives of the invention. Such modifications are contemplated as within the scope of the claimed invention. 

1. A method, comprising: performing a shunt measurement with one non-inductive shunt value using an excitation signal including a root mean squared current and a frequency range; capturing a response time record of said shunt measurement; transforming said response time record of said shunt measurement to a frequency domain; normalizing said response time record of said shunt measurement transformed to said frequency domain to said one non-inductive shunt value; and recording said response time record of said shunt measurement transformed to said frequency domain and normalized to said non-inductive shunt value as a calibration record; performing a device measurement using said excitation signal including said root mean squared current and said frequency range; capturing a response time record of said device measurement; transforming said time record of said device measurement to said frequency domain; applying said calibration record to said response time record of said device measurement; and generating a measure of said device.
 2. The method of claim 1, further comprising: performing said shunt measurement using only a high range root mean squared current level; and scaling said shunt measurement based on said shunt measurement using only said high range root mean square current level.
 3. The method of claim 1, further comprising: performing said shunt measurement with said one non-inductive shunt value using an excitation signal including said root mean squared current and each of a plurality of frequency ranges; capturing said response time record of said shunt measurement including each one of said plurality of frequency ranges; transforming said response time record of said shunt measurement including each one of said plurality of frequency ranges to said frequency domain; normalizing said response time record of said shunt measurement including each one of said plurality of frequency ranges transformed to said frequency domain to said one non-inductive shunt value; recording said response time record of said shunt measurement including each one of said plurality of frequency ranges transformed to said frequency domain and normalized to said non-inductive shunt value as a plurality of calibration records; performing said device measurement at one of said plurality of frequency ranges; capturing said response time record of said device measurement at said one of said plurality of frequency ranges; transforming said response time record of said device measurement at said one of said plurality of frequency ranges at said high range root mean square current level; selecting one of said plurality of calibration records corresponding to said one of said plurality of frequencies used in said device measurement; and applying said one of said plurality of calibration records to said response time record of said device measurement.
 4. The method of claim 3, wherein said plurality of frequency ranges comprise harmonic octave subsets of said frequency range.
 5. The method of claim 4, wherein said harmonic octave subsets comprise exact harmonic octave subsets of said frequency range.
 6. The method of claim 1, further comprising: recording said response time record including a negative time period from time zero, said negative time period corresponding to a fraction of a period of a lowest frequency of said excitation signal; and discarding said negative time portion of said response time record.
 7. The method of claim 6, wherein said fraction of said period of said lowest frequency comprises about ten percent of said period of said lowest frequency.
 8. The method of claim 1, further comprising: analyzing said response time record; determining time periods in said response time record where a voltage level exceeds a saturation level of a digitizer within a data acquisition system; discarding said time periods in said response time record where said voltage level exceeds said saturation level of said digitizer; discarding said time periods in said calibration record which correspond to said time periods discarded in said response time record; applying said calibration record having said time periods discarded to said time response record having said time periods discarded; and generating said measure of said device. 